Base station modulator/demodulator and ATM cell send/receive method

ABSTRACT

A base station modulator/demodulator for constituting a mobile communication system and sending ATM cells to a higher rank station and receiving ATM cells from the higher rank station is provided. The base station modulator/demodulator comprises: receive means for terminating, among ATM cells received from the higher rank station through a leased line, ATM cells addressed to the base station; first send means for sending, among the ATM cells received from the higher rank station through the leased line, ATM cells addressed to another base station to the another base station; and second send means for multiplexing ATM cells of a plurality of base stations, the base station and the another base station, and sending the multiplexed cells to the higher rank station through the leased line. By virtue of the above construction, for a base station wherein the usable band of the ATM cell between the base station and the higher rank station has been reduced due to a reduction in size/reduction in capacity of the base station, ATM cells of a plurality of base stations are logically multiplexed in a leased line between the base station and the higher rank station, the master base station terminates the ATM cell addressed thereto, and, at the same time, the send of ATM cell to a slave base station and the receive of ATM cell from the slave base station can be realized, whereby the waste of the leased line can be eliminated.

FIELD OF THE INVENTION

[0001] The invention relates to a base station modulator/demodulator andan ATM (asynchronous transfer mode) cell send/receive method utilizingan ATM line, and more particularly to a base stationmodulator/demodulator and an ATM cell send/receive method suitable forthe elimination of the waste of a leased line at the time ofsend/receive of ATM cells.

BACKGROUND OF THE INVENTION

[0002] There is a conventional mobile communication system whichperforms send/receive of ATM cells utilizing an ATM line between ahigher rank station and a base station.

[0003]FIG. 1 is a diagram showing an example of the construction of aconventional mobile communication system. In this mobile communicationsystem, a base station A 1 and a base station B 2 are connected to ahigher rank station through a network 3. The network 3 is connected tothe base station through a leased line. Trailing data from the higherrank station is sent to the base station A 1 through a leased line andto the base station B 2 through another leased line. Data received bythe base station A is designated as trailing ATM cell A 4, and data sentfrom the base station A to the higher rank station is designated asleading ATM cell A 6. Likewise, for the base station B, data received bythe base station B is designated as trailing ATM cell B 5, and data sentfrom the base station B to the higher rank station is designated asleading ATM cell B 7.

[0004]FIG. 6 shows an ATM cell format. An ATM cell 25 is constituted bydata of 53 bytes. In this case, 5 bytes from the head constitute an ATMheader 26, and the remaining 48 bytes constitute a payload 27. The ATMheader 26 comprises GFC (generic flow control) 28, VPI (virtual pathidentification) 29, VCI (virtual channel identification) 30, PT (payloadtype) 31, CLP (cell loss priority) 32, and HEC (header error control)33.

[0005] GFC 28 is provided for flow control which is performed whentraffic has been increased and, consequently, overload state has takenplace over the network 3. VPI 29 is utilized in setting of a virtualpath between the higher rank station and the base station, and VCI 30 isutilized for the identification of each of a plurality of data in theset VP (virtual path) in the communication of the plurality of data. PT31 indicates the state of cell (congestion), and CLP 32 indicates thesignificance of cell. HEC 33 functions to detect bit error of the ATMheader 26, and indicates the results of coding of 8-bit CRC for 4 bytesin the header except for HEC 33. A payload 27 indicates the storageregion of communication data.

[0006]FIG. 7 is a diagram showing a leased line (a secondary group)frame format as one example of the leased line frame format. Thesecondary group of leased lines have a transmission capacity of 6.3 Mbpswherein 789 bits are arranged in a frame of time length 125 μs. In thisformat, 8 bit-length 98 TSs (time slots) can be obtained. In a portionof 96 TSs (96 bytes×8=768 bits) among them, ATM cells 25 arecontinuously arranged.

[0007]FIG. 2 shows an example of the construction of a conventionaltrailing ATM cell. The trailing ATM A 4 from the higher rank station isreceived in an HWY interface section 8 in the base station A 1. Areceive processor 35 has the function of terminating the trailing ATMcell A 4. The base station A 1 and the base station B 2 are handled asdevices which are independent of each other. Thus, these base stationshave the same construction.

[0008]FIG. 3 shows an example of the construction of a conventionaltrailing processing function. This function will be explained by takingthe base station A 1 as an example. For data received from the leasedline, the physical layer is terminated in the HWY interface section 8.In a leased line frame trailer 11, the frame format shown in FIG. 7 issynchronized. After the establishment of the synchronization of theframe, cell synchronization for establishing the position of cellboundary is carried out in an ATM cell synchronous detector 12, foridentifying an ATM cell 25 mapped within the frame format.

[0009] For the ATM cell 25 within the frame format which has been pickedout in the ATM cell synchronous detector 12, error detection of HEC 33is carried out in the ATM HEC error detector 22, and the ATM cell 25,which has been found to have an error, is discarded in this function.The trailing ATM cell A 4. which has been judged to be effective in thephysical layer is sent to the receive processor 35. In the receiveprocessor 35, VPI 29 is confirmed for the trailing ATM cell A 4 receivedby VPI filter 14, and only the trailing ATM cell A 4 having VPI 29assigned to the base station A 1 is transferred to the next processing.The ATM cell 25, wherein VPI 29 is different from the set value, isdiscarded by this function.

[0010] The trailing ATM cell A 4, which has been passed through thefilter of VPI 14, judges various ATM cells assigned by the VCI filter15, and is terminated at an ATM cell trailer 16.

[0011]FIG. 4 shows an example of the construction of a conventionalleading ATM cell. The leading ATM cell A 6 from the base station A isgenerated in an ATM cell generator 20. Since the base station A 1 andthe higher rank station are connected to each other through a leasedline, the base station A 1 maps the leading ATM cell B 7 in the leasedline (secondary group) frame format shown in FIG. 7 through the use of aframe generator 23. Further, there is also a function of mapping of anidol cell 34 in the frame format according to the transmission ratecapacity of the leading ATM cell A 6.

[0012] Regarding another example of the conventional mobilecommunication system, for example, Japanese Patent No. 3003779 proposesa mobile communication system wherein the control link between anexchange and a radio base station is established using a control signalby ATM.

[0013] The above-described conventional prior art techniques, however,had the following problems.

[0014] The first problem is such that when reduction in size/reductionin capacity of the base station has lead to a reduction in quantity ofdata for communication between the base station and the higher rankstation and, consequently, the quantity of data has become smaller thanthe capacity of the leased line, in the mapping within the leased line,the proportion of the effective cell becomes larger than the proportionof the idol cell. This creates wasteful line charge. When the number ofusers to be supported by the base station has been changed, for example,due to a reduction in size, changing the rate including the quantity ofdata in the cable to an optimal rate is preferred. Since, however, thereis no leased line suitable for the capacity, the system of the basestation should be constructed in such a state that an unused bandexists.

[0015] The second problem is such that, in solving the first problem,the connection of the conventional model should be possible. That is, asystem should be devised which has succeeded to the conventional systemdesign and can utilize existing models.

SUMMARY OF THE INVENTION

[0016] Accordingly, it is an object of the invention to provide a basestation modulator/demodulator and an ATM cell send/receive method,wherein, for a base station in which the band of ATM cell utilizedbetween the base station and the higher rank station has been reduceddue to a reduction in size/reduction in capacity, ATM cellscorresponding to a plurality of base stations are logically multiplexedwith the leased line between the base station and the higher rankstation and the termination of the ATM cell addressed to itself in themaster base station and the send of ATM cell to a slave base station andthe receive of ATM cell from the slave base station can besimultaneously realized to enable the elimination of the waste of theleased line.

[0017] According to the first feature of the invention, a base stationmodulator/demodulator for constituting a mobile communication system andsending ATM cells to a higher rank station and receiving ATM cells fromthe higher rank station, comprises:

[0018] receive means for terminating, among ATM cells received from thehigher rank station through a leased line, ATM cells addressed to thebase station;

[0019] first send means for sending, among the ATM cells received fromthe higher rank station through the leased line, ATM cells addressed toanother base station to the another base station: and

[0020] second send means for multiplexing ATM cells of a plurality ofbase stations, said base station and said another base station, andsending the multiplexed cells to the higher rank station through theleased line.

[0021] According to the second feature of the invention, an ATM cellsend/receive method in a mobile communication system for performingsend/receive of ATM cells between a higher rank station and a basestation, comprises the steps of:

[0022] terminating, among ATM cells received from the higher rankstation through a leased line, ATM cells addressed to the base station;

[0023] sending, among the ATM cells received from the higher rankstation through the leased line, ATM cells addressed to another basestation to the another base station; and

[0024] multiplexing ATM cells of a plurality of base stations, said basestation and said another base station, and sending the multiplexed cellsto the higher rank station through the leased line.

[0025] Referring now to FIGS. 8 and 9, the base stationmodulator/demodulator according to the invention is provided in a basestation constituting a mobile communication system and sends ATM cellsto a higher rank station and receives ATM cells from the higher rankstation. The base station modulator/demodulator comprises: receive means(9) for terminating, among ATM cells received from the higher rankstation through a leased line, ATM cells addressed to the base station;first send means (10) for sending, among the ATM cells received from thehigher rank station through the leased line, ATM cells addressed toanother base station to the another base station; and second send means(19) for multiplexing ATM cells of a plurality of base stations, thebase station and the another base station, and sending the multiplexedcells to the higher rank station through the leased line.

[0026] According to the base station modulator/demodulator of theinvention, for a base station wherein the usable band of the ATM cellbetween the base station and the higher rank station has been reduceddue to a reduction in size/reduction in capacity of the base station,ATM cells of a plurality of base stations are logically multiplexed in aleased line between the base station and the higher rank station, themaster base station terminates the ATM cell addressed thereto, and, atthe same time, the send of ATM cell to a slave base station and thereceive of ATM cell from the slave base station can be realized, wherebythe waste of the leased line can be eliminated. Further, since themaster base station and the slave base station have the same processingfunction, the thinking of “master/slave base station” is unnecessaryand, thus, this facilitates system design. Furthermore, since the masterbase station has the same processing function as the slave base station,a further slave base station can be easily provided under the slave basestation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention will be explained in more detail in conjunctionwith the appended drawings, wherein:

[0028]FIG. 1 is a block diagram showing an example of the constructionof a conventional mobile communication system;

[0029]FIG. 2 is a block diagram showing the construction of a trailingATM cell in the conventional mobile communication system;

[0030]FIG. 3 is a block diagram showing the construction of a trailingprocessing function of the conventional mobile communication system;

[0031]FIG. 4 is a block diagram showing the construction of a leadingATM cell of the conventional mobile communication system;

[0032]FIG. 5 is a block diagram showing the construction of a mobilecommunication system according to a preferred embodiment of theinvention;

[0033]FIG. 6 is an explanatory view showing the construction of an ATMcell format according to a preferred embodiment of the invention;

[0034]FIG. 7 is an explanatory view showing the construction of a leasedline (secondary group) frame format according to a preferred embodimentof the invention;

[0035]FIG. 8 is a block diagram showing the construction of a trailingATM cell according to a preferred embodiment of the invention;

[0036]FIG. 9 is a block diagram showing the construction of a leadingATM cell according to a preferred embodiment of the invention;

[0037]FIG. 10 is a block diagram showing the construction of a trailingprocessing function according to a preferred embodiment of theinvention; and

[0038]FIG. 11 is a block diagram showing the construction of a leadingprocessing function according to a preferred embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Preferred embodiments of the invention will be explained indetail in conjunction with the accompanying drawings.

[0040] (1) Explanation of construction

[0041] In data communication between a base station and a higher rankstation in a mobile communication system, a transmission method is usedwherein an existing leased line is used as a PM (physical media)sublayer and an ATM cell has been mapped in a leased line frame format.

[0042] As shown in FIG. 5, the mobile communication system according toa preferred embodiment of the invention generally comprises a basestation A 1, a base station B 2, and a network 3.

[0043] The above construction will be explained in detail. The basestation A 1 is connected to a higher rank station through a network 3.The network 3 and the base station A 1 are connected to each otherthrough a leased line. An ATM cell 25 (see FIG. 6) for the base stationA 1 and the base station B 2 is multiplexed in the trailing data fromthe higher rank station. The trailing ATM cell for the base station B 2separated in the base station A 1 is sent from the base station A 1 tothe base station B 2. The leading ATM cell sent from the base station B2 is received in the base station A 1, is multiplexed in the leading ATMcell of the base station A 1, and is then sent to the higher rankstation.

[0044]FIG. 6 shows the ATM cell format. The ATM cell 25 is constitutedby data of 53 bytes, 5 bytes from the head are accounted for by an ATMheader 26, and the remaining 48 bytes constitute a payload 27. The ATMheader 26 consists of GFC 28, VPI 29, VCI 30, PT 31, CLP 32, and HEC 33.

[0045] GFC 28 is provided for flow control which is performed whentraffic has been increased and, consequently, overload state has takenplace over the network 3. VPI 29 is utilized in setting of a virtualpath between the higher rank station and the base station, and VCI 30 isutilized for the identification of each of a plurality of data in theset VP (virtual path) in the communication of the plurality of data. PT31 indicates the state of cell (congestion), and CLP 32 indicates thesignificance of cell. HEC 33 functions to detect bit error of the ATMheader 26, and indicates the results of coding of 8-bit CRC for 4 bytesin the header,except for HEC 33. A payload 27 indicates the storageregion of communication data.

[0046]FIG. 7 is a diagram showing a leased line (a secondary group)frame format as one example of the leased line frame format. Thesecondary group of leased lines have a transmission capacity of 6.3 Mbpswherein 789 bits are arranged in a frame of time length 125 μs. In thisformat, 8 bit-length 98 TSs (time slots) can be obtained. In a portionof 96 TSs (96 bytes×8=768 bits) among them, ATM cells 25 arecontinuously arranged.

[0047]FIG. 8 shows the construction of a trailing ATM cell according toa preferred embodiment of the invention. The trailing ATM cell from thehigher rank station is received in an HWY interface section 8 of thebase station A 1. A master receive processor 9 terminates a trailing ATMcell A 4 addressed to the base station A 1 as the master. A slave sendprocessor 10 identifies a trailing ATM cell B 5 addressed to the basestation B 2, and sends the trailing ATM cell B 5 to the base station B2. The base station B 2 has the same construction as the base station A1, and, in order to receive the ATM cell 25 from the base station A 1,has the HWY interface section 8 and a master receive processor 9.

[0048]FIG. 9 shows the construction of a leading ATM cell according to apreferred embodiment of the invention. A leading ATM cell A 6 from thebase station A 1 to the higher rank station is generated in an ATM cellgenerator 20, and a leading ATM cell B 7 in the base station B 2 isgenerated in the slave send processor 22. The base station A 1 receivesthe leading ATM cell B 7 from the base station B 2 in a slave interfacesection 21, multiplexes both the leading ATM cells 25 in a cellmultiplexer 19, followed by the send of the multiplexed cells to aleased line.

[0049] Specifically, according to the base station connection methodutilizing an ATM line according to a preferred embodiment of theinvention, in a base station modulator/demodulator (base station) in amobile communication system for data communication between a basestation and a higher rank station by a transmission method wherein anATM cell is mapped utilizing an existing leased line as a PM (physicalmedia) sublayer, when the quantity of data utilized in one base stationis much smaller than the band of the leased line connected due to areduction in size/reduction in capacity of the base station, data of aplurality of base stations are multiplexed through an ATM layer in theleased line and the master base station functions to send data to theslave base station, to receive data from the slave base station, and toperform multiplex sending of the received data and the self-stationdata, whereby the effective utilization of the leased line is realizedand the extension of the slave base station can be easily realized.

[0050] (2) Explanation of operation

[0051] The operation of the preferred embodiment of the invention willbe explained in detail in conjunction with FIGS. 6 to 11. As shown inFIG. 8, the construction of the trailing ATM cell according to thepreferred embodiment of the invention is such that the trailing ATM cellfrom the higher rank station is received in the state of multiplexing ofthe ATM cell for the base station A 1 and the ATM cell for the basestation B 2. Since the trailing ATM cell A 4 and the trailing ATM cell B5 are mapped in the same leased line, in the case of VP connectionwherein the cells are judged by path identification, the cells should bedifferent from each other in VPI 29 in the ATM header 26 shown in FIG.6.

[0052] The master receive processor 9 and the slave send processor 10recognize VPI 29 set in the base station A 1 and the base station B 2.

[0053] The construction of a trailing processing function according tothe invention is as shown in FIG. 10. A leased line (secondary group)frame format as an example of the leased line frame format is shown inFIG. 7 described above.

[0054] The secondary group leased line has a transmission capacity of6.3 Mbps wherein 789 bits are arranged in a frame of time length 125 μs.In this format, 98 TSs (time slots) of 8 bit-length can be obtained. Ina portion of 96 TSs (96 bytes ×8=768 bits) among them, ATM cells 10 arecontinuously arranged. Since the length of the ATM cell 25 is 424 bits(53 bytes), as shown in the drawing, in some cases, the boundary of the125 μs frame does not confirm to the cell boundary and, in this case,one ATM cell 25 extends over two frames. The specifications are inaccordance with ITU-T (International TelecommunicationUnion-Telecommunication Standardization Sector) G. 804.

[0055] For the data received from the leased line, the physical layer isterminated in the HWY interface section 8. The trailing data receivedfrom the leased line is subjected to the synchronization of the frameformat shown in FIG. 7 in the leased line frame terminal 8. Further,after the establishment of the frame synchronization, cellsynchronization for establishing cell boundary position is carried outin an ATM cell synchronous detector 12 to identify the ATM cell 25mapped within the frame format.

[0056] The ATM cell 25 within the frame format, which has been pickedout in the ATM cell synchronous detector 12, is subjected to errordetection of HEC 33 by an ATM HEC error detector 13, and ATM cell 25having an error is discarded in this function. The trailing ATM cell inthe physical layer judged to be effective is simultaneously sent to themaster receive processor 9 and the slave send processor 10.

[0057] In the master receive processor 9, for the received trailing ATMcell, VPI 29 is confirmed by a VPI filter 14, and only the trailing ATMcell A 4 having VPI 29 assigned to the base station A 1 as the master istransferred to next processing. The trailing ATM cell B 5 having VPI 29different from the set value is discarded by the present function. Thetrailing ATM cell A 4, which has been passed through the filter of VPI14, judges various ATM cells 25 assigned by a VCI filter 15, followed bytermination in an ATM cell terminator 16.

[0058] In the slave send processor 10, as with the master receiveprocessor 9, the VPI filter 14 is used to judge only the trailing ATMcell B 5 assigned to the base station B. Since the trailing ATM cell B Shas been sent from the higher rank station through the leased line inthe state of multiplexing with the ATM cell A 4 of the base station A,behind the VPI filter 14, the band of the ATM cell B 5 is smaller thanthe band of the leased line. For this reason, a speed regulator 17inserts an idol cell 34 instead of the ATM cell 25 discarded in the VPIfilter 14.

[0059] The base station A 1 and the base station B 2 are connected toeach other through a cable, and conversion to a leased line (secondarygroup) frame format is carried out in a frame generator 18 of a slavesend processor 10. The base station B 2 has the same trailing processingfunction as the base station A 1. and the use of only the master receiveprocessing can realize the termination of the trailing ATM cell B 5.Further, when the base station B 2 has the same trailing processingfunction as the base station A 1, the whole system can be constructed byan identical hardware. This can easily realize the provision of a basestation C in the slave of the base station B 2.

[0060]FIG. 9 shows the construction of a leading ATM cell according to apreferred embodiment of the invention, and FIG. 11 shows theconstruction of a leading processing function according to a preferredembodiment of the invention.

[0061] The leading ATM cell B 7 from the base station B 2 is generatedin the ATM cell generator 20. The base station B 2 is connected to thebase station A 1 through a cable, and the base station B 2 uses a framegenerator 23 of a cell multiplexer 19 to map the leading ATM cell B 7 inthe leased line (secondary group) frame format shown in FIG. 7. The basestation B 2 further has the function of mapping an idol cell 34 in theframe format according to the transmission rate capacity of the leadingATM cell B 7.

[0062] Regarding the leading signal from the base station B 2, thephysical layer is terminated in the slave interface section 21 of thebase station A 1. The slave interface section 21 functions in the samemanner as the HWY interface section 8 which terminates the trailingphysical layer. The leading ATM cell B 7 withdrawn in the slaveinterface section 21 is transferred to the cell multiplexer 19. Further,in the base station A 1, the ATM cell A 6 generated in the ATM cellgenerator 20 is transferred to the cell multiplexer 19. The cellmultiplexer 19 comprises an ATM cell multiplexer 24 and a framegenerator 23.

[0063] When the ATM cell A 6 and the ATM cell B 7 introducedrespectively from the base station A 1 and the base station B 2 are sentto the leased line, the ATM cell multiplexer 24 controls the number oftimes of send. For each leased line used between the higher rank stationand each base station, the band is previously determined. Therefore, theband is controlled by varying the number of times of send of each ATMcell according to the set value. The ATM cell 25 assigned in the ATMcell multiplexer 24 generates a frame format for a leased line in theframe generator 23.

[0064] The base station B 2 has the same leading processing function asthe base station A 1, and the use of only the ATM cell generator 20 canrealize the generation of the leading ATM cell B 7. Further, when thebase station B 2 has the same leading processing function as the basestation A 1, the whole system can be constructed by an identicalhardware. This can easily realize the provision of a base station C inthe slave of the base station B 2.

[0065] As described above, according to the preferred embodiment of theinvention, for a base station wherein the usable band of the ATM cellbetween the base station and the higher rank station has been reduceddue to a reduction in size/reduction in capacity of the base station,ATM cells of a plurality of base stations are logically multiplexed in aleased line between the base station and the higher rank station, themaster base station terminates the ATM cell addressed thereto, and, atthe same time, the send of ATM cell to a slave base station and thereceive of ATM cell from the slave base station eliminated.

[0066] Since the master base station and the slave base station have thesame processing function, the thinking of “master/slave base station” isunnecessary and, thus, this facilitates system design.

[0067] Further, since the master base station has the same processingfunction as the slave base station, a further slave base station can beeasily provided under the slave base station. [Other preferredembodiments]

[0068] In the above preferred embodiment, the higher rank station andthe base station A are connected to each other through a leased line,and the master base station and the slave base station are connected toeach other through a cable. In another preferred embodiment, a leasedline may also be used for connection between the master base station andthe slave base station. In this case, although only the secondary group(6.3 M) leased line is used in the above preferred embodiment, inanother preferred embodiment, a more cost-effective system may beconsidered wherein, for the slave base station, conversion from thesecondary group to the primary group is made.

[0069] Further, in the above preferred embodiment, an existing leasedline is utilized. The same technique can also be applied to an SDH(synchronous digital hierarchy) system, for example, utilizing 155 M,and rate down from 155 M to the secondary group is possible.

[0070] The provision of a plurality of slave send functions within thebase station permits the connection of the plurality of slave basestations under the master base station.

[0071] As is apparent from the foregoing description, the base stationmodulator/demodulator according to the invention performs the followingcontrol. Among ATM cells received through a leased line from a higherrank station, ATM cells addressed to a base station are terminated, andATM cells addressed to another base station among the ATM cells receivedthrough the leased line from the higher rank station are sent to theanother base station. Further, ATM cells of a plurality of basestations, a base station and other base station, are multiplexed, andthe multiplexed cells are sent to the higher rank station through aleased line. In addition, the base station provided with the basestation modulator/demodulator has the same processing function as otherbase station. The above construction can offer the following effects.

[0072] The first effect is such that, for a base station wherein theusable band of the ATM cell between the base station and the higher rankstation has been reduced due to a reduction in size/reduction incapacity of the base station, ATM cells of a plurality of base stationsare logically multiplexed in a leased line between the base station andthe higher rank station, the master base station terminates the ATM celladdressed thereto, and, at the same time, the send of ATM cell to aslave base station and the receive of ATM cell from the slave basestation can be realized, whereby the waste of the leased line can beeliminated.

[0073] The second effect is such that, since the master base station andthe slave base station have the same processing function, the thinkingof “master/slave base station” is unnecessary and, thus, thisfacilitates system design.

[0074] The third effect is such that, since the master base station hasthe same processing function as the slave base station, a further slavebase station can be easily provided under the slave base station.

[0075] The invention has been described in detail with particularreference to preferred embodiments, but it will be understood thatvariations and modifications can be effected within the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A base station modulator/demodulator forconstituting a mobile communication system and sending ATM cells to ahigher rank station and receiving ATM cells from the higher rankstation, said base station modulator/demodulator comprising: receivemeans for terminating, among ATM cells received from the higher rankstation through a leased line, ATM cells addressed to the base station;first send means for sending, among the ATM cells received from thehigher rank station through the leased line, ATM cells addressed toanother base station to the another base station; and second send meansfor multiplexing ATM cells of a plurality of base stations, said basestation and said another base station, and sending the multiplexed cellsto the higher rank station through the leased line.
 2. The base stationmodulator/demodulator according to claim 1, wherein the receive meansidentifies, from ATM cells received from the higher rank station throughthe leased line, ATM cells addressed to said base station based on avirtual path identifier in an ATM header preset in the base station, andterminates the identified ATM cells, and the first send meansidentifies, from ATM cells received from the higher rank station throughthe leased line, ATM cells addressed to said another base station basedon a virtual path identifier in an ATM header preset in the another basestation, and sends the identified ATM cells to the another base station.3. The base station modulator/demodulator according to claim 1 or 2,which further comprises: discard means for discarding, among the ATMcells received by the receive means from the higher rank station throughthe leased line, ATM cells having a virtual path identifier differentfrom the set value; and insertion means for inserting an idol cellinstead of the ATM cell discarded by the discard means.
 4. The basestation modulator/demodulator according to claim 1 or 2, wherein thesecond send means performs band control in such a manner that, inmultiplexing ATM cells of the plurality of base stations, the basestation and the another base station, and sending the multiplexed cellsthrough the leased line to the higher rank station, the number of timesof send of the ATM cell of the base station and the ATM cell of theanother base station is varied based on the band set value predeterminedfor the leased line.
 5. The base station modulator/demodulator accordingto any one of claims 1, 2, and 4, wherein the base station has the sameprocessing function as the another base station.
 6. An ATM cellsend/receive method in a mobile communication system for performingsend/receive of ATM cells between a higher rank station and a basestation, said method comprising the steps of: terminating, among ATMcells received from the higher rank station through a leased line, ATMcells addressed to the base station; sending, among the ATM cellsreceived from the higher rank station through the leased line, ATM cellsaddressed to another base station to the another base station; andmultiplexing ATM cells of a plurality of base stations, said basestation and said another base station, and sending the multiplexed cellsto the higher rank station through the leased line.